1. Field of the Invention
This invention relates generally to refrigeration systems, and more particularly to energy conservation arrangements for refrigeration systems in which a plurality of compressors are operated independently and randomly.
2. History of the Prior Art
Hotels, restaurants, supermarkets, and the like, use many individual refrigeration units which must be separately controlled. In a supermarket, for example, the food freezers must be kept below a given maximum temperature, the refrigerator cabinets must be kept below a higher maximum temperature, and lesser degrees of cooling are used for other special purposes, such as providing additional cooling in the fruits and vegetables department. In restaurants, there are not only frozen food lockers and refrigerators, but also special air conditioning needs for special areas, such as the kitchen. During operation, therefore, refrigeration capacity may be needed at a particular point for a period of time, and then an idle time may ensue before temperature again must be brought down. The problems vary greatly, dependent on outside temperature and internal conditions, and a number of approaches have been devised for meeting the various refrigeration needs. Usually, the refrigeration system, which may be one large unit or a number of individual units, is mounted outside the building, and sensors control switching of the unit on and off as needed.
Saving energy is an important consideration in these systems, because proper usage of refrigeration capacity can effect substantial savings under typical conditions. It is often required, for example, to supply heat to increase the interior temperature of an establishment while running the refrigeration system, so that the two energy consuming systems work against each other. A number of computer controlled systems have been devised for improving the cycling of integrated systems, but the refrigeration system can be treated separately, in that it is desirable to reduce the amount of energy required to deliver refrigeration as needed to a number of different operative units.
Among known units are the multiple-compressor type of refrigeration systems, in which an exterior refrigeration unit, with large cooling fans, is mounted outside the restaurant or store, with individual compressors operating independently as needed by the associated refrigeration units within the store. Typically, freezers and coolers must be operative for, say, 18 hours in a day, and run for about 40-45 minutes per hour in a random fashion. In a multiple compressor system, therefore, one or more compressors are running at practically all times, even when the ambient temperature is extremely cold.
Refrigeration compressors dissipate a considerable amount of heat energy during normal operation, and this heat must be carried off either by ambient air or by other means to prevent the compressors from overheating. If overheating occurs, the compressor is shut down and refrigeration capacity is not available. While this problem occurs primarily during warm weather, it also may occur in cooler weather under certain operating conditions.
In response to this problem, some systems have used individual fans for the separate compressors, these fans drawing considerable amperage and substantially adding to the energy costs.
One specific technique that has been used for cooling compressors only is to wrap a copper coil about the compressor and circulate water through it, in a conventional heat exchange relation. The system condenser itself, however, is water-cooled and, thus there is no energy saving, although there is protection against compressor overheating.
A separate problem arises during cold weather, when the refrigeration requirements are satisfied and the compressor cycles off. It may remain off for a long enough period to cool down below the source temperature, which causes refrigerant to be drawn into the compressor, diluting the compressor lubricating oil. The diluted oil foams excessively on restart of the compressor, providing poor lubrication and excessive oil loss into the system. This loss of oil may leave the compressor with insufficient oil for lubrication, precipitating early compressor failure.
U.S. Pat. No. 4,411,141, issued Oct. 25, 1983, to A. Hara, discloses a parallel operation compressor in refrigeration apparatus where lubricant is forced from the crankcase of one compressor to another compressor. If this approach were used in a multi-compressor system as discussed above, however, it would result in a very expensive unit requiring specially constructed compressors.